1. Field of the Invention
This invention relates to an absorption type multi-layer film ND filter (neutral density filter) having a substrate and an optical multi-layer member consisting of metal layers and dielectric layers which are alternately layered on the substrate. More particularly, it relates to an improvement of an absorption type multi-layer film ND filter, so improved as to be able to achieve flat transmittance characteristics in a better reproducibility and moreover have superior mass productivity.
2. Description of the Related Art
ND filters refer to optical filters having non-selective transmittance such that, of light rays, those belonging to all wavelengths in the visible spectral range are substantially equally transmitted, and are used in the state they are fitted to lenses of digital cameras or the like in order to attenuate the quantity of transmitted light. For example, they are commonly used in a case in which, when films are inevitably overexposed however lenses are stopped down under conditions where the quantity of light is large as in fine weather, the quantity of light is limited so that the shutter can be released at a lower speed, or in a case in which, when the diaphragm should be set full-open but films are inevitably overexposed even though the shutter is released at a maximum speed, the quantity of light is limited so that the diaphragm can be set full-open.
Now, inexpensive ND filters include glass filters formed of glass to which a light absorber has been added. However, such glass filters have had problems such that their spectral characteristics are not uniform over the whole visible range.
As what can solve such a problem, the absorption type multi-layer film ND filter is known in the art. For example, Japanese Patent Application Laid-open No. S57-195207 discloses a light absorbing film in which two multi-layer films having dielectric films and metal films and having reflection preventive effect even when they are each used alone are so put together from above and below as to face each other with other dielectric film inside at the middle.
As background art making reference to the types of materials for dielectric films and metal films, Japanese Patent Publication No. S55-47361 for example also discloses an absorptive thin film formed of Ti or Cr metal films and MgF2 dielectric films which are alternately layered. Still also, Japanese Patent Application Laid-open No. 2002-350610 discloses a thin-film type ND filter formed of Nb metal films and SiO2 dielectric films which are alternately layered.
Japanese Patent Application Laid-open No. H7-63915 further discloses a thin-film type ND filter which has a multi-layer structure having about seven layers as the number of layers and making use of light absorptive metal oxide layers including those of titanium oxides such as TiO and TiO2.
However, in the light absorbing film disclosed in Japanese Patent Application Laid-open No. S57-195207, only two layers are used as the metal films and hence there has been a problem that, depending on the type of the material to be used as a metal film, it is very difficult to make small the wavelength dependence of transmittance and also control reflectance. There has further been a problem that, since the metal film is a film which is as thin as several nanometers and is readily oxidized, the transmittance may come higher than the designed value because of thermal oxidation in the course of a manufacturing process.
In the case of the absorptive thin film or thin-film type ND filter disclosed in Japanese Patent Publication No. S55-47361 or Japanese Patent Application Laid-open No. 2002-350610, there has been a problem that the wavelength dependence of transmittance in the visible spectral range, of the metal film constituted of Ti, Cr or Nb is so great as to make it difficult to achieve the transmittance attenuation that is flat for wavelengths. There has also been a problem that the material MgF2 used in the dielectric film may greatly corrode or deteriorate by water adsorption in the air.
In addition, in the thin-film type ND filter disclosed in Japanese Patent Application Laid-open No. H7-63915, the light absorptive metal oxide layers including those of titanium oxides are formed by vacuum deposition. There, however, has been a problem that, in forming metal oxide layers (TiO, TiO2) as intermediate layers by vacuum deposition or sputtering, the flow rate of oxygen must delicately be controlled in either of the vacuum deposition and the sputtering, making it difficult to stabilize film quality.